Friction stir welding (FSW) is a relatively new welding process for joining together parts of materials such as metals, plastics, and other materials that will soften and commingle under applied frictional heat to become integrally connected. A detailed description of an FSW apparatus and process may be found in Patent Publications WO 93/10935 and WO 95/26254; and U.S. Pat. No. 5,460,317, all of which are hereby fully incorporated by reference. One useful apparatus for FSW is shown in FIGS. 1A and 1B and includes a shoulder 14' at its distal end, and a nonconsumable welding pin 16' extending downward centrally from the shoulder. As shown, two parts to be welded together, exemplified by plates 10A', and 10B' on backing plate 12' are aligned so that edges of the plates along the weld joint are in direct contact. As the rotating tool W' is brought into contact with the weld interface between plates 10A' and 10B', the rotating pin 16' is forced into contact with the material of both plates, as shown. The rotation of the pin in the material and rubbing of the shoulder against the upper surface of the material produce a large amount of frictional heating of both the welding tool and the plate interface. This heat softens the material of the plates in the vicinity of the rotating pin and shoulder, causing commingling of material which, upon hardening, forms a weld. The tool is moved longitudinally along the interface between plates 10A' and 10B', thereby forming an elongate weld all along the interface between the plates. When the weld is completed, the welding tool is retracted.
Still referring to FIGS. 1A and 1B, as the FSW pin 16' plunges into the interface between plates 10A' and 10B', the downward pressure of the pin 16' tends to force plates 10A' and 10B' apart. Separation of the plates during welding may cause anomalies in the weld, such as a surface trough void or a subsurface "wormhole" type of void. Currently, this separation is mitigated by bolting a strap over each plate at the start point of the weld. For this purpose, a special run-on tab must be provided on each plate at the start point of the weld to receive the strap. The strap holds the run-on tab and the backing plate together, and serves to oppose separation of the plates caused by forcing the tool between the plates. Accordingly, this method requires tooling to secure the plates together, in addition to extra straps and run-on tabs. Furthermore, this procedure is time consuming as it involves drilling of the backing plate 12' and bolting the strap in place. The drilling is also damaging to the backing plate 12'. A need exists to provide a new joint weld design and method for eliminating the plate joint separation that does not require additional materials or tooling, is not time consuming, and does not damage the backing plate. In particular, a need exists to provide a joint weld design and method for eliminating plate separation that does not require run-on tabs, since some workpieces cannot have run-on tabs.